This invention generally relates to devices for detecting radioactive contamination in hand-holdable objects, such as the tools that are used to service a nuclear power facility.
Devices for detecting radioactive contamination in small objects are known in the prior art. Such devices are generally known in the art as "friskers," and are often used to check whether or not a tool or other object which has been subjected to a decontamination process has in fact been rendered completely free of such radioactive contaminants. Generally, such devices comprise a radiation detector (which may be either a gas-flow proportional detector or a scintillation-type detector) mounted on a short, table-like structure. The tools or other hand-held objects to be tested are placed over the radiation detector, which scans them for radioactive contaminants. In some such devices, a second radiation detector is disposed over the first detector, so that radiation readings from both the bottom and top surfaces of the tool or other object may be taken simultaneously. Such a dual-detector configuration has the advantage of detecting contaminants which emit radiation from only one side of the tool, as can happen when a radioactive particle is lodged within a crevice of the tool. The successful operation of such radiation detection devices is important, since the failure of such a device to detect the presence of radioactive contaminants could result in the lodgment of radioactive particles in the skin of one of the facility operators.
While there are frisker-type radiation detectors in the prior art which are generally capable of satisfactorially determining whether or not a particular tool or other object emits an unacceptably high amount of radioactivity, the applicants have observed a number of shortcomings in these prior art devices. One such shortcoming is the manner in which these devices solve the problem of preventing the radiation alarm circuits from being spuriously actuated by background radiation. This is a serious problem as such frisker-type detectors are often operated in or adjacent to the decontamination rooms of nuclear facilities, where tools and other objects awaiting decontamination radiate a significant amount of gamma radiation throughout the room. To prevent such spurious triggering of radiation alarm circuitry, some of these devices exclusively rely upon a microprocessor which has been programmed to periodically sample the background gamma radiation, and to subtract the sampled background radiation value from the readings obtained from the frisker-type radiation detector as the objects are being examined thereby. While exclusive reliance upon "background subtraction" obviates the need for providing a thick and heavy lead shield around the radiation detector to block out the background radiation, it can also cause the device to give inaccurate or false readings since background gamma radiation in a nuclear facility is very much subject to considerable, moment-to-moment fluctuations caused by the movement of contaminated equipment in or around the detector of the device. To overcome the shortcomings associated with the exclusive reliance upon background subtraction, a few prior art designs provide partial shielding around the radiation detector. However, the shielding in the hoods of every such detector that the applicants are aware of affords such incomplete protection so as to only marginally reduce the dependency upon "background subtraction."
Other shortcomings associated with many of the prior art frisker-type radiation detector devices are the result of the type of radiation detectors used in such devices. Single-zone gas-flow proportional detectors are incapable of informing the operator whether the radiation emitted by the tool or other object is the result of a single, localized "hot particle," or is the result of a contaminant that is uniformly spread over the surface of the tool. Still another problem associated with single-zone gas-flow proportional detectors is the relatively low signal to noise ratio that such detectors yield when placed in a substantial field of background radiation. Some prior art designs have attempted to remedy the deficiencies associated with single-zone detection by providing a bank of separate, scintillation-type detectors. However, such detector banks are apt to have "dead zones" in certain areas between adjacent detectors which are blind to radiation, thereby affording an opportunity for a "hot particle" to escape detection. Additionally, the fragility of the thin plastic "windows" used in such detectors makes them very easy to break when a hard, heavy and sharp object is placed on them. Attempts have been made to solve the fragility problem by using thicker windows of plastic. However, the use of such thick plastic panes desensitizes the detectors to beta radiation, thereby forcing the detector to rely exclusively upon its sensitivity to gamma radiation in making its measurements. Such exclusive reliance upon gamma radiation disadvantageously decreases the signal to noise ratio of the detector and greatly increases the time necessary for the detectors to determine the amount of radiation emitted by the tool being scanned.
Finally, many of the frisker-type detectors of the prior art must be manually actuated prior to and during operation by the manipulation of buttons on a control panel. Applicants have observed that the button configurations in such control panels often provides a situs where radioactive debris and airborne particles can contaminate the device.
Clearly, what is needed is a frisker-type radiation detector apparatus which is capable of accurately, reliably and consistently detecting the presence of radioactive material on tools and other objects. Ideally, such a radiation detector should have a plurality of mutually-contiguous or overlapping radiation-sensitive zones so that the existence and location of one or more "hot particles" in a particular tool or other object may be reliably determined. Additionally, the device should have some sort of means for cancelling out the effects of background radiation on the detector which minimizes reliance upon computerized "background subtraction." Finally, it would be desirable if such a device could be easily operated without the need for manipulating manual controls which, as pointed out previously, can afford a situs of potential radiation contamination.